Optical emission spectroscopy (OES) is a mature, robust technology for the elemental analysis of materials. For metals, the prevalent techniques for generating an emission spectra use either an electric arc or a spark, or both, (collectively hereinafter “spark”) to vaporize a small quantity of a sample to be analyzed. A survey of OES analytical techniques may be found in Slickers, Automatic Atomic-Emission Spectroscopy, Second Edition (1993), which is incorporated herein by reference.
The basic elements of the OES method are described with respect to FIGS. 1 and 2. An electrically conducting sample 100 is placed beneath an electrode 102. An arc spark 104, initiated by an appropriate electronic circuit 106, with a return path 108, vaporizes a small quantity of metal at a highly localized point 110 where the spark 104 strikes the sample 100. Monochromatic visible and/or invisible (i.e., infrared and/or ultraviolet) photo emissions 200 (FIG. 2) characteristic of elements in the vaporized metal are captured by a light guide 202, which sends the light to a spectrometer 204, which analyzes the spectrum so as to yield the elemental composition.
In order to be confident that the composition deduced from a measurement, which typically tests a miniscule portion of the sample 100, is representative of the composition of the entire sample 100, minimizing effects from, for example, inclusions, matrixes and surface contaminants, it is standard practice to average the spectra from as many as several thousand sparks that have struck an area as large as 100 square mm in the few seconds of the measurement. This large number of spark strikes typically leaves a blackened area of strike points.
Advanced techniques record and analyze the individual spectra from the thousands of sparks of a single measurement so as to eliminate outlying spectra and obtain the most confident compositional analysis of the bulk of the sample 100. At this time, however, no method exists for determining where an individual spark strikes the sample 100 and, hence, the position of an individual spark cannot be correlated with the elemental composition of the material excited by the spark.